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Segment And Geographic Information (Segment Table) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total net revenues	$ 1,702	$ 1,396	$ 1,189
Income before income taxes	1,049	761	458
Assets	61,162	54,673	48,734
Electronic Brokerage [Member]
Segment Reporting Information [Line Items]
Total net revenues	1,405	1,239	1,097
Income before income taxes	860	756	536
Assets	58,787	50,072	44,421
Market Making [Member]
Segment Reporting Information [Line Items]
Total net revenues	86	190	298
Income before income taxes	(27)	44	130
Assets	8,469	11,765	10,825
Corporate And Eliminations [Member]
Segment Reporting Information [Line Items]
Total net revenues	211	(33)	(206)
Income before income taxes	216	(39)	(208)
Assets	$ (6,094)	$ (7,164)	$ (6,512)

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Section S99
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 25
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-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41242-110953

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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